Too Early To Spray Fungicide On Corn?

Some growers include a fungicide in the spray tank when making their second application of glyphosate herbicide on corn. They want to control foliar diseases and weeds in one shot, but ISU plant pathologists don't recommend it.

Is now too early to spray a fungicide to control foliar diseases of corn? "I think it is," says Alison Robertson, Iowa State University Extension plant pathologist. She acknowledges that it might be appealing for a corn grower to add a fungicide into the spray tank when making a second round of glyphosate herbicide application to kill weeds, especially at today's high grain prices.

The idea of using a fungicide-herbicide tank mix is to try to boost yield by providing control of corn leaf diseases, and to save time by doing the two jobs (disease control and weed control) in one trip across the field. "However, this is not something that is generally recommended by university-based plant pathologists," says Robertson.

With an early application of fungicide, the timing isn't quite right for effective disease control. You are better off waiting until later to apply the fungicide. Robertson explains more about early season fungicide applications in this week's "Crop Minute" audio file available from ISU Extension. You can listen to it by clicking on http://www.agron.iastate.edu/cropminute/.

You are better off waiting until tasseling time to apply fungicide

Earlier this week Robertson spent time assessing her fungicide trials in central Iowa. "In this test, we apply our first application of fungicide at the corn growth stage of V5 to V6," she says. "This week, prior to spraying, I noticed anthracnose leaf spot was present on the first two leaves on the corn plant but at very low severity. I also saw a few eyespot lesions on the second leaf of a scattering of plants. An application of fungicide will not cure these infected leaves, but could protect other leaves from infection."

Remember however, most of the leaves are tightly wound up in the whorl of the developing plant. "Only those leaf surfaces that are directly contacted by the fungicide will be protected," she notes. "The leaves we need to protect — the ear leaf and those above — are not exposed to benefit from a V5 application. Also remember that most fungicides are only effective for approximately 21 days after spraying. Thus, an application at V5 would not protect your crop throughout the season, even if all leaves did come into contact with the product. If you do choose to apply a fungicide, an application at tasseling or soon thereafter, is probably more prudent."

"Severe thunderstorms with hail have already caused damage to crops this growing season," says Roberton. "ISU Extension agronomist Roger Elmore and I have both gotten questions from farmers about applying a fungicide to these hail-affected corn fields. There are no clear data on the benefits of an application of fungicide to hail-damaged crops."

Robertson says in a simulated hail-fungicide trial, University of Illinois plant pathologist Carl Bradley found no yield benefit. Robertson adds, "If you do decide to spray a hail-damaged field with a fungicide, be sure to leave a couple untreated strips across the field and take careful notes of crop development and yield to determine if the application was beneficial."